很多人都接触过文件上传和下载，比如JavaWeb。
在这里我记录一下在Spring框架中的此类基本操作，水平有限，不喜勿喷。

首先编写好前端ftl格式的界面

<p>单文件上传</p>
<form action="upload" method="POST" enctype="multipart/form-data">
    文件：<input type="file" name="file">
    <input type="submit">
</form>

<p>文件下载</p>
<p><a href="download">下载文件</a></p>

<p>多文件上传</p>
<form action="batch" method="POST" enctype="multipart/form-data">
    <p>文件1：<input type="file" name="file"></p>
    <p>文件2：<input type="file" name="file"></p>
    <p><input type="submit" value="上传"></p>
</form>
</body>

配置对外访问路径fileUpload

@GetMapping("/fileUpload")
public String fileUpload(ModelMap modelMap){
    
    modelMap.addAttribute("msg", "文件上传下载");
    return "fileUpload";
}

对于单文件上传操作，主要通过流的方式，把前端上传的文件写入到服务端指定位置。对于File中的transferTo方法，我是试了一下发现上传的文件内容为空，不知道问题出在什么地方

@RequestMapping(value = "/upload")
public String upload(@RequestParam("file")MultipartFile file){
    
    if (file.isEmpty()){
    
        return "file is null";
    }
    String fileName = file.getOriginalFilename();
    log.info("upload filename: "+ fileName);

    File dest = new File(filePath);
    if (!dest.exists()){
    
        dest.mkdirs();
    }
    try {
    
        BufferedOutputStream bos = new BufferedOutputStream(new FileOutputStream(new File(dest,fileName)));
        byte[] bytes = file.getBytes();
        bos.write(bytes);
        bos.close();
        return "upload ok";
    } catch (IOException e) {
    
        e.printStackTrace();
    }
    return "upload wrong";
}

多文件上传操作类似于单文件上传操作，是单文件上传的叠加，也是通过流的方式。

@PostMapping("/batch")
public String handleFileUpload(HttpServletRequest request){
    
    List<MultipartFile> files = ((MultipartHttpServletRequest)request).getFiles("file");
    MultipartFile file = null;
    BufferedOutputStream stream = null;
    for (int i = 0; i < files.size(); i++) {
    
        file = files.get(i);
        if (!file.isEmpty()){
    
            try {
    
                byte[] bytes = file.getBytes();
                stream = new BufferedOutputStream(new FileOutputStream(new File(filePath+file.getOriginalFilename())));
                stream.write(bytes);
                stream.close();
            } catch (IOException e) {
    
                stream = null;
                return "number "+i+" upload wrong "+e.getMessage();
            }
        }else {
    
            return "number "+i+" upload wrong , because file is null";
        }
    }
    return "upload ok";
}

我这边是硬编码指定了一个下载文件，对于下载操作，要设置好响应头中的项，比如文件名，文件长度等

@GetMapping("/download")
public String downloadFile(HttpServletResponse response){
    
    String fileName = "readme.txt";
    if (fileName != null){
    
        File file = new File(filePath+fileName);
        if (file.exists()){
    
            response.setContentType("application/force-download");//强制下载不打开
            response.addHeader("Content-Disposition", "attachment;fileName="+fileName);//文件名
            response.setContentLength((int)file.length());
            response.setCharacterEncoding("utf-8");

            byte[] buffer = new byte[1024];
            FileInputStream fis = null;
            BufferedInputStream bis = null;
            try {
    
                bis = new BufferedInputStream(new FileInputStream(file));
                OutputStream os = response.getOutputStream();
                int i = bis.read(buffer);
                while (i != -1){
    
                    os.write(buffer, 0, i);
                    i = bis.read(buffer);
                }
                return "down ok";
            } catch (Exception e) {
    
                e.printStackTrace();
            }finally {
    
                if (bis != null){
    
                    try {
    
                        bis.close();
                    } catch (IOException e) {
    
                        e.printStackTrace();
                    }
                }
                if (fis != null){
    
                    try {
    
                        fis.close();
                    } catch (IOException e) {
    
                        e.printStackTrace();
                    }
                }
            }
        }
    }
    return "down wrong";
}